Stirring it up

By Abbe Miller

October 2009- The external tank is the largest of the space shuttle's fuel tanks. And when the shuttle is on the launch pad, awaiting its powerful lunge toward space, it almost appears to be hugging the large red rocket, which supports the craft before liftoff. The external tank, or ET, can be described as the gas tank for the shuttle as it holds the bulk of the propellant needed to boost the shuttle into orbit.

The propellant mixture is made up of liquid oxygen and liquid hydrogen. Together their weight totals 1,585,379 lbs. In volume, they total 526,126 gallons.

A tight shipIn recent years NASA decided to lightweight the ET. Just as automobile makers have striven to slash their weights substantially, so too did Lockheed Martin Corp., Bethesda, Md., the manufacturer of the ET. Constructed at the Michoud assembly facility in New Orleans, the ET was originally composed of aluminum alloy 2219. But as further research and development occurred with new space alloys, it adopted 2195, shedding 7,500 lbs..

This new material, although lightweight and strong, posed new problems for welding its panels together. Repair welds were now more difficult and the joint strength had decreased. The problem was solved, however, with friction stir welding, a process that's discussed in FFJournal'sOctober article, "New frontiers," which covers the construction of NASA's new space vehicle, the Orion.

According to an article released by Lockheed Martin, "friction stir welding is the most recent upgrade to the space shuttle's gigantic external tank, the largest element of the space shuttle and the only element not reusable. The new welding technique--being marketed to industry--utilizes frictional heating combined with forging pressure to produce high-strength bonds virtually free of defects. Friction stir welding transforms the metals from a solid state into a 'plastic-like' state and then mechanically stirs the materials together under pressure to form a welded joint."

Bigger and betterOnce the process was perfected, it was incorporated into the ET that was used on STS-128, the most recent launch. The ET, with its new, lighter body and its tight welds took orbit on Aug. 28 for the international space station. In addition to its improved metallurgical benefits, the friction stir weld is also easier to complete than the previous fusion weld process. Rotation speed, travel speed and pressure are three of many process variables that have now been upgraded.

As for the ET, it's a pity that its legacy can't be preserved along with the other reusable portions of a launch--the shuttle and its solid rocket boosters. According to NASA's Web site, "When the solid rocket boosters separate at an altitude of approximately 45 km (28 miles), the orbiter, with the main engines still burning, carries the external tank piggyback to near orbital velocity, approximately 113 km (70 miles) above the Earth. The now-nearly empty tank separates and falls in a preplanned trajectory with the majority of it disintegrating in the atmosphere and the rest falling into the ocean." MM